Technologies using an unmanned aerial vehicle (UAV) in surveying are publicly known. The UAV used in these technologies may be mounted with a location measuring device using a global navigation satellite system (GNSS) (what is called a “GPS receiver”), an inertial measurement unit (IMU), an altimeter, and a camera. Such a UAV may be made to photograph the ground while flying along a predetermined path in aerial photogrammetry or in other surveying.
Photogrammetry greatly depends on accuracy of location data of the camera. The UAV can locate its own location using the GNSS, but the locating accuracy is approximately 1 meter in a horizontal direction and is approximately 3 meters in a vertical direction, which does not satisfy the accuracy required in the photogrammetry. Mounting of a more highly accurate location measuring device using a GNSS on the UAV may be considered, but this idea is difficult to apply to a general purpose UAV in consideration of weight and electric power consumption of the device. To solve these problems, a total station (TS) may be used to track a UAV in flight and locate the UAV by using its laser distance measuring function (for example, refer to US2014/0210663).
This method of tracking the UAV by the TS uses an automatic target-tracking function of the TS. In this technique, scanning laser light is used to capture and track the UAV. The UAV has a reflective prism that reflects the scanning laser light back in the incident direction, and the TS detects light reflected from the reflective prism to track the UAV.
The UAV can move unpredictably due to winds and other factors, and the TS may lose sight of the UAV. The TS may also lose sight of the UAV while tracking the UAV due to obstacles, such as tree branches, leaves, birds, utility poles, and electric lines, between the TS and the UAV.